Jig grinding and jig boring deliver a high degree of accuracy and repeatability ideal for the moldmaking industry. Unfortunately, as accurate as these processes are, they have several drawbacks. Both are costly, requiring heavy initial capital investment and are labor intensive. They're also less flexible and require knowledgeable operators to work the machinery, and such individuals can be hard to find.

Despite these limitations, jig processing will always have a place in moldmaking. Its accuracy and repeatability are proven, and for certain applications, it is the only process that will deliver the required results. Fortunately, advancements in high-speed machining technologies now provide mold makers with an alternative in applications where jig grinding and jig boring are frequently used.

To illustrate how the processes compare, Makino conducted a trial using a jig-bore machine and a V33i vertical machining center. Both machines milled a D2 steel sample (60 Rockwell HRc), a typical material used in die and mold applications, measuring 6 inches square by 1 inch deep. The detail consisted of 20 round-through holes 20mm in diameter (approximately 0.75 of an inch) with a hole pitch measuring 25mm (approximately 1 inch).

"We selected this configuration because a true round circle is one of the most difficult details to machine," says Keith Roeser, application engineer with Makino. "Furthermore, machining 20 identical holes of consistent size, roundness and location, which is required in a die plate or mold base, is nearly impossible."

Preparing the Samples[back to top]
The same roughing and semi-finishing process was used on all three samples. After these techniques were complete, one sample was finished using a jig-bore machine, and the other two were finished on the V33i, using a plunge method and a helix method.

"We used a standard Z-level roughing process from our CAM system," explains Roeser. "The tool used was an OSG high-feed mill that was 10mm in diameter and had a 2.0mm corner radius. The cut was performed in a V33i machining center with a 30,000-RPM HSK63F spindle."

The part featured a down step of 0.0118 inches with a step-over of 0.118 inches, a standard for D2 material. The initial cut was performed at 2,500 RPM at a feedrate of 118 IPM, which resulted in a tool life of 13 holes. The spindle was then set to 3,000 RPM at a feedrate of 118 IPM. In this instance, the tool completed 18 holes. Next, the spindle was set to 3,000 RPM with an increased feedrate of 142 IPM, which machined all 20 holes with a 51-minute cycle time, or 2.5 minutes per hole.

A Hitachi TH-coated 10mm-high helix six-flute tool was used during the semi-finishing process. This method minimized the interrupted cutting caused by other styles of tooling. The tool was modified slightly by clearing back the flutes by 0.004 of an inch per side except for the last 5mm of flutes. This step minimized the re-cutting or vibration of the flutes hitting the sidewalls that had already been machined.

"The semi-finish process stepped the material out, taking two 0.008 of an inch widths of cuts and stepping down 3mm in a helix motion," says Roeser. "This process took the remaining stock down from 0.020 of an inch to 0.004 of an inch per side."

Finishing on a Jig-Bore Machine[back to top]
The jig-bore machine used to finish the sample was equipped with a 40,000-RPM air turbine spindle with a Nasa CNC control. The programming was performed on the control, which was very similar to Fanuc G and M code.

"During the jig-bore finishing process, all 20 holes were bored with a Borazon wheel. This left a rougher finish, but it removed more material than a grit wheel and didn't break down as often," explains Roeser. "The Borazon wheel left only 0.0001 of an inch of material per side in all 20 holes, a process that took about 20 seconds per hole per pass."

Next, a 60-grit wheel, which was dressed similar to a surface grinder, was installed in the jig-bore machine. The tool was about 0.625 of an inch in diameter and had a 0.25 of an inch of side cutting surface. This method was used to machine the remaining 0.0001 inch of stock out of the holes.

"This tool ran at 40,000 RPM, and the outer head spun at 240 RPM. The operator used a cylindrical cutting motion for this geometry," says Roeser. "The tool required dressing three times to make sure the holes were the same size, which is considered normal for this type of material at 60 HRc."

The finishing cycle time on the jig-bore machine was one hour and 40 minutes. It took an additional 50 minutes to set up the part and program, for a total of two hours and 30 minutes. The total machining time from roughing to finish was two hours and 50 minutes. The surface finish varied from 10.5 Ra to 18.2 Ra micro inches.

The X and Y locations were measured at the top, middle and bottom of the holes to determine error. All 60 locations had a combined error of 0.00021 of an inch in the X-axis and 0.00017 of an inch in the Y-axis. Further measurement indicated all 20 holes were less than 0.0002 of an inch from the desired diameter of 20mm at each of the three levels. The roundness of all 20 holes varied anywhere from +0.0003 to -0.0003 of an inch.

V33i Plunge Machining[back to top]
Roeser used a plunge method for the first of two finishing methods on the V33i. During the plunge method, the tool cuts in the Z-negative direction, moves away from the material, rises up and locates the next position, and performs another Z-negative plunge.

The same type of tool used in the semi-finish process was used. The V33i ran at 2,000 RPM with a 40-ipm feed rate. The step-over was 0.004 inch.

"One of the unique benefits of plunge machining is that mold makers can use the tool's radius to get the cusp height," says Roeser. "This offers the ability to take a much larger step-over and still leave a small cusp height."

The cycle time for the plunge machining on the V33i was 15 minutes per hole, which totaled five hours of cutting. Considering the step-over was 0.004 of an inch on a 0.787-inch hole, the machine had to plunge 618 times for each hole. The surface finish varied from 11 to 23 Ra micro inches. The X-axis had a combined error of 0.00029 of an inch, and the Y-axis measured 0.00035 of an inch over all 20 holes.

"This might seem excessive, but when you consider this machine is locating a total of 12,368 times to plunge out all 20 holes, it's pretty amazing," says Roeser. "In comparison, the jig borer only repositioned 20 times."

During plunge machining, the diameter variance of all 20 holes measured 0.0023 of an inch, and the variance on roundness ranged between 0.0002 of an inch to 0.00014 of an inch.

V33i Helix Finishing[back to top]

The last process used was a helix finish method, in which all three axes moved in unison. This process removed the remaining 0.004 of an inch of stock in one pass. The cycle time was much faster than the plunging method at only 45 minutes, or two minutes and 15 seconds per hole.

"During this process, the V33i ran at 1,910 RPMs at 587mm per minute, or 23 inches per minute. The step-down was 0.040 inch continuous through the block," explains Roeser.

The cycle time for helix machining was 45 minutes, the fastest of all three methods. The next closest was the jig bore at one hour and 40 minutes. The surface finish varied from 4.0 to 8.3 Ra micro inches.

The helix finishing method had the least error. The X-axis had a combined error of 0.00007 inch, and the Y-axis 0.00016 inch over all 20 holes. The holes were all within +/- 0.0001 inch to each other over all 60 positions. The roundness of the hole was good, with a deviation of no more than 0.0002 of an inch.

Comparing the Results[back to top]
As displayed in charts on the right, all machining methods produced similar diameter, roundness and locations. In these charts, each hole has been arranged clockwise by machining order with deviations displayed radially. Comparison measurements were taken from the top of every hole.

The V33i helix method performed best with regard to diameter deviation. Throughout all 20 holes, an average deviation of less than 0.0002 of an inch was held on the required 20mm diameters.

In regard to roundness characteristics, the V33i plunge method produced the most accurate results with an average roundness deviation of 0.00008 of an inch. This method was followed closely by the V33i helix method at 0.0002 of an inch. Lastly, for location, the V33i helix method proved the most repeatable, with less than 0.0002 of an inch total deviation in any location of all 20 holes.

"The V33i helix and plunge methods have performed up to or better than jig grinding in most hole characteristics, providing a viable alternative to work typically reserved for jig grinding," says Roeser.

Faster, Flexible and Cost-Effective[back to top]
Jig grinding and boring are time-tested processes and are critical for delivering the accuracy mold makers demand. As accurate as the processes are, slow cycle times, high manufacturing costs and limited tool, coolant and fixture options hamper mold makers that use jig grinding or jig boring.

Recent tests indicate that high-speed machining on vertical machining centers offers a viable alternative for mold makers that don't want to sacrifice cost, speed and flexibility for accuracy.

"Some manufacturers are using the V33i's high-speed machining capabilities to reduce cycle times by as much as 75 percent compared to jig grinding, while still achieving the desired accuracy," says Roeser.

Certain jobs still demand jig grinding, but now mold makers have a new option with the V33i.

For more information on the V33i, visit www.makino.com/v33i.